1. Field of the Invention
The present invention relates to liquid crystal display devices, methods for controlling the devices, and portable terminals, and in particular, to an active-matrix liquid crystal display device including active elements for pixels, a method for controlling the liquid crystal display device when it is in a power-off state, and a portable terminal in which the liquid crystal display device is used as a screen display unit.
2. Description of the Related Art
When a liquid crystal display device is switched off (in a power-off state), residual electric charge in pixels may cause a residual image forming distortions on the screen.
A method for shutting off the supply of power to a liquid crystal panel has been employed as a measure in the related art for preventing screen distortions occurring in the power-off state. In this method, in response to a power-off command issued when a user operates a power-on/off button, white data is written in all pixels in the case of a normally-white liquid crystal display device, or black data is written in all pixels in the case of a normally-black liquid crystal display device, whereby the pixels are controlled to display white or black so that screen distortions are eliminated. After that, by turning off a power-supply switch provided on a power-supply line, the supply of power to the liquid crystal panel is shut off.
In this method, writing of the white data or black data is sequentially performed in units of rows by a scanning operation, as in the case of ordinary writing of display data, and writing of the white data or black data for one screen requires a minimum of one field period. Thus, this method cannot cope with a sudden occurrence of the power-off state, which is an instantaneous event. The sudden occurrence of the power-off state includes, for example, a case in which a user mistakenly or deliberately removes a power-supply battery from a portable terminal (e.g., a cellular phone) whose screen display unit is a liquid crystal display device.